1. Field of the Invention
The present invention relates to an evacuating valve attached to a vacuum vessel and connected to a vacuum pump for use in evacuating the vacuum vessel. More specifically, the present invention relates to an evacuating valve to be used in combination with a vacuum vessel in which an atmosphere of an ionized gas (hereinafter, referred to as “plasma atmosphere”) is created.
2. Description of the Related Art
Referring to FIG. 5, a known evacuating valve 50 of this kind has a chamber port structure 52 to be connected to a vacuum vessel 51, a pump port structure 53 to be connected to a vacuum pump, and a valve mechanism 55 placed in a passage 54 between the chamber port structure 52 and the pump port structure 53. The valve mechanism 55 includes a valve seat 56 placed in the passage 54, a valve element 57 to be seated on the valve seat 56 to close the evacuating valve 50 and to be separated from the valve seat 56 to open the evacuating valve 50, and a sealing member 58 put on the valve element 57 to create a nonleaking union between the valve seat 56 and the valve element 57. The valve element 57 is separated from the valve seat 56 to evacuate the vacuum vessel 51. The valve element 57 is seated on the valve seat 56 to maintain a predetermined vacuum in the vacuum vessel 51.
Prior art relevant to the present invention is disclosed in, for example, JP-A No. 9-137879.
When the vacuum vessel 51 is used for a semiconductor device fabricating process, the vacuum vessel 51 is evacuated to a predetermined vacuum, and a plasma atmosphere is created in the vacuum vessel 51. As shown in FIG. 5, the valve element 57 of the known evacuating valve 50 has a substantially flat sealing surface 57a facing the chamber port structure 52, and the sealing member 58 is fitted in a circular groove formed in a peripheral part of the valve element 57. When the evacuating valve 50 is connected to the vacuum vessel 51 in which a plasma atmosphere is created, the chamber port structure is always open into the vacuum vessel 51, the chamber port structure 52 is filled up with the plasma and the sealing member 58 is always exposed to the plasma atmosphere. Consequently, the deterioration of the sealing member 58 of the valve mechanism is accelerated by the collision of active ionized molecules of gases against the sealing member 58, i.e., by plasma attack.